Interpretive Summary: Application of fertilizer nitrogen in corn production systems generally increases grain yield and aboveground production of crop residue. It is a widely-held belief that greater crop residue production translates to higher soil organic matter levels over the long term. Our data from long-term experimental sites demonstrates that this paradigm does not adequately capture the effects of fertilizer N additions on soil C dynamics in conventioally managed corn-soybean rotations in Iowa. We found that despite increased crop residue amounts, amounts of soil organic carbon remained unchanged with N fertilization, even after >30 yr at high N-addition rates. These results suggest that N fertilization actually increases decomposition of soil organic matter over the long-term. This information will be useful to scientists seeking to understand the effects of N fertilizer on soil organic matter dynamics and the impact of agricultural practices on carbon storage in soils.

Technical Abstract:
Management practices in croplands have the potential to influence the global carbon budget via effects on C storage in soils. Under the current paradigm, N fertilization is credited with increasing crop biomass production, and thus inputs of organic C to soils, which in turn increase soil C sequestration. Our data from long-term experimental sites demonstrate that this paradigm does not capture the effects of N additions on C dynamics in the dominant rain-fed agroecosystem of the Midwestern USA, rotations of maize (Zea mays L.) and soybean (Glycine max (L.) Merr.) under conventional tillage. We found that under increasing N fertilization, total organic C inputs increased, but relatively less than did grain yield, by a factor of 0.28-0.98. This unpredictability was attributed to variability in root production. Despite increasing organic C inputs, soil organic carbon stocks remained unchanged with N fertilization, even after >30 yr at high N-addition rates. Stimulation of organic C decay rate by N fertilization is the only plausible explanation for lack of net effect of N fertilizer additions on soil C sequestration.